1. Field of the Invention
The present invention relates to semiconductor memories, and more particular to MOS semiconductor memories occupying a small area and yet having a great capacitance.
2. Description of the Prior Art
Dynamic random access memories (DRAMs) comprising one transistor and one capacitor are known as the simplest DRAMs. With such memories, the amount of charge in the capacitor is altered by turning on and off a gate by the transistor to store data in the desired memory cell.
FIG. 3 is a sectional diagram showing a stacked DRAM cell. The stacked DRAM cell has a capacitor wherein a thin film of great dielectric constant is held between electrodes to give an increased capacitance per unit area.
With reference to the diagram, indicated at 20 is a silicon substrate formed with a device isolation region 28. With a device forming region thereby defined, the silicon substrate 20 is formed with a gate electrode 23 with an oxide film 22 provided therebetween. The gate electrode 23 is provided as a word line over the silicon substrate 20 to form a select transistor. Source/drain regions 21 are formed in a surface layer of the silicon substrate 20 at opposite ends of the gate electrode 23. A word line 24 for an adjacent cell is provided on the device isolation region 28. A node electrode 25 in contact with one of the source/drain regions 21 extends over the gate electrode 23, over this source/drain region 21 and over the word line 24. A plate electrode 27 is formed over the node electrode 25 with a dielectric 26 interposed therebetween. A bit line 30 is further provided over the plate electrode 27, with an interlayer insulating film 29 formed therebetween, and is connected to the other source/drain region 21 which is not in contact with the node electrode 25.
To hold the electric charge stored in the capacitor of the semiconductor memory described, there is a need to give increased capacitance to the capacitor. However, an increase in the capacitance of the cell is in conflict with a reduction in the cell size, so that attempts to reduce the cell size encounter the problem of necessitating ingenuity in various aspects.